The present invention relates to a family of titanium-based intermetallic alloys which combine a number of specific mechanical properties comprising high yield stress, high creep strength and sufficient ductility at ambient temperature.
Intermetallic alloys of the Ti.sub.3 Al type have been found to exhibit useful specific mechanical properties. Ternary alloys with additions of Nb in particular have been tested and their mechanical properties, combined with a lower density than that of nickel-based alloys (typically between 4 and 5.5 depending on the Nb content) have aroused great interest for aeronautical applications. These alloys furthermore have a greater titanium fire resistance than the Ti-based alloys used previously in the construction of turbomachines. The applications envisaged involve solid structural components such as casings, solid rotating components such as centrifugal impellers, or as a matrix for composites for integrally bladed rings. The desired service temperature ranges are up to 650.degree. C. or 700.degree. C. in the case of components made of a long-fiber composite.
U.S. Pat. No. 4,292,077 and U.S. Pat. No. 4,716,020 describe the results obtained from titanium-based intermetallic alloys containing from 24 to 27% Al and from 11 to 16% Nb in at %.
U.S. Pat. No. 5,032,357 has shown improved results by increasing the Nb content. In this case, the intermetallic alloys obtained generally have a microstructure composed of two phases:
a niobium-rich B2 phase forming the matrix of the material and providing ductility at ambient temperature; and PA1 a so-called O phase, with the defined composition Ti.sub.2 AlNb, which is orthorhombic and forms lamellae in the B2 matrix. The O phase is present up to 1000.degree. C. and gives the material its hot strength properties in creep and in tension. PA1 Al, from 16 to 26; Nb, from 18 to 28; Mo, from O to 2; Si, from O to 0.8; Ta, from O to 2; Zr, from O to 2; and Ti as the balance to 100; with the condition that Mo+Si+Zr+Ta&gt;0.4%.
However, these known prior alloys have certain drawbacks, particularly an insufficient ductility at ambient temperature and extensive plastic strain during primary creep, which at the present time limit their use.